The technology of plastic laser transmission welding is known from a number of publications, for instance EP 1 575 756 B1 or DE 10 2004 036 576 B4.
Generally speaking, a joining member which is transmissive of the laser processing radiation is placed on a joining member which absorbs said radiation. When a corresponding joining zone between the two joining members is exposed to radiation emitted through the transmissive joining member, a melt is produced in the absorptive joining member. Due to the contact with the transmissive joining member, a corresponding amount of heat energy is transported into the transmissive joining member, causing the transmissive joining member to melt in the welding zone as well, with the result that a corresponding mixing of the materials takes place between the two joining members. When the materials have cooled, a weld seam is formed between the two joining members which weld seam has a contour in the manner of a nugget in a direction transverse to the weld seam direction. Because of the different refractive powers of the two joining members and of the mixed material in the region of the weld seam, the interface of the nugget contour relative to its surroundings in the joining members is an optically active interface which is able to reflect a light beam.
A method for producing and monitoring a weld seam by means of laser radiation according to DE 101 21 923 C2 takes advantage of this feature in such a way that a radiation, which is referred to as control radiation in this disclosure, is emitted into a measuring zone comprising the weld seam, and the measuring radiation reflected by the interface is transmitted to a detection unit for detection.
This prior art monitoring procedure carried out on the weld seam only allows one to determine disturbances in the detected reflected measuring radiation caused by defects in the weld seam, which is supposed to trigger reactions. A reaction of this type may be the rejection of a workpiece that is determined to be defective.